FIELD OF THE INVENTION
This invention relates to a spark plug in which an insulator is supported by resting its stepped portion on a shoulder portion of a metallic shell.
In a spark plug for an internal combustion engine, an insulator has generally been made of aluminum oxide (Al.sub.2 O.sub.3) which is inferior in heat-conductivity.
With high speed and high power requirement of the internal combustion engine, a front end of the insulator tends to be exposed to higher ambient temperature so as to cause preignition. In order to prevent the spark plug against the preignition, it is suggested in an integral type of the spark plug that the insulator is integrally made of aluminum nitride (AlN) superior in heat-conductivity so as to prevent preignition within an extensive range of running condition of the internal combustion engine.
In Provisional Patent Application No. 2-183985 filed on Jan. 6, 1989 which was laid open on Jul. 18, 1990 and was published on Jun. 8, 1994 as JP 6-44504, the applicant of the invention had suggested a split-type insulator which has a rear half made of aluminum oxide and a front half made of aluminum nitride, and the rear half is joined to the front half.
In the integral type of the spark plug, a shortage of withstand voltage of the insulator causes a spark discharge to penetrate across the insulator so as to ensue a misfire when a high tension is applied across the spark plug. This is because the aluminum nitride (AlN) is inferior to the aluminum oxide (Al.sub.2 O.sub.3) in withstanding high voltage. It is very often that the spark discharge penetrates across the portion in which the insulator is next to or opposite the shoulder portion of the metallic shell, since the stepped portion of the insulator rests on the shoulder portion.
In the split-type insulator, the spark discharge frequently penetrates across the front half of the insulator from the reason that the shoulder portion of the metallic shell directly comes across an entire section of the front half made of aluminum nitride.